Would anyone care to comment on this?
I agree with Don and Bookbndrbob.
Truthfulness and honesty would / should have the vendor mentioning this in
However, one must give leeway to perchance the vendor is ignorant of the etiquette, (as you are) with experience of selling and the intricacies of the varied desires of the purchaser.
It would not worry me a jot, if I received "mint" stamps with catalogue numbers on the back, but that is just my feeling, with experience with years on this forum, one becomes aware of the varied rigid desires of some purchasers.
What then, if you desired perfectly centred stamps?
If you intend to continue with this determination of perfectly "mint" stamps, I'd suggest you email the vendors beforehand, outlining your desires, that should prevent any further regret for you..
In the meantime, I would put down your latest purchase to "experience"
To expect everyone to have the same standards as oneself, is a sure route to suffering.